This invention relates in general to the art of coating and in particular to an improved method for carrying out a process of coating in which one or more layers of coating composition, preferably a conductive composition, are applied to the surface of a substrate by advancing the substrate through a coating zone in which a flow coating composition is applied thereto, for example, a process of bead coating or a process of curtain coating. More specifically, this invention relates to an improved coating method in the manufacturing of a photographic film, photographic paper, photographic printing layer, a magnetic recording tape, an adhesive tape, pressure-sensitive recording layer, an offset printing plate material or the like.
A method of applying an electrostatic force to assist in a coating method, along with a conventional method of coating a continuously moving web, has been previously disclosed. For example, as disclosed by Hartman in WO 89/05477, ionizers may be used to deposit polar charge on the web prior to the coating application locus to generate an electrostatic field at the coating application locus for a curtain coating method. This electrostatic assist enables the coating method to operate at increased speeds without the defect of air bubbles trapped in the coating layers or between the web and the coated layer. Many prior patents are cited by Hartman discussing the use of polar charge assist in a bead coating method, as well as methods of measuring and controlling the electrostatic field so that a uniform charge of the required magnitude is obtained. These patents do not describe any particular electrical properties of the web that are particularly helpful to the use of electrostatic assist for a coating method.
In another example disclosed by De Geest in U.S. Pat. No. 3,335,026, a potential difference is applied between the coating roller and the coating composition to generate an electrostatic field to attract the coating composition to the web. This patent constrains the resistivity of the web surface to be greater than 10.sup.9 ohms/square. However, as is shown below in the description of the present invention, it is not the surface resistivity alone, but its combination with the web speed and web capacitance while on the coating roller that determines the effectiveness of the electrostatic assist. Thus, it is possible to use electrostatic assist for web surfaces having a resistivity less than 10.sup.9 ohms/square. Furthermore, De Geest does not address the issue of designing a support with respect to surface resistivity and web capacitance so as to achieve a specified coating speed using electrostatic assist with minimized coating roller voltage levels. By minimized coating roller voltage levels it is meant that the voltage level is preferably as close as possible to the voltage level required when using an insulating web having a surface resistivity greater than 10.sup.13 ohms/square.
In another example disclosed by Nakajima in U.S. Pat. No. 4,837,045, an electrostatic force on a coating composition is combined with a web having a gelatin-subbing layer containing a surfactant. This electrostatic force allows an increase in speed of coating without increasing the load of drying the coated layers. The gelatin-subbing layer is required to contain a surfactant to achieve the desired electrostatic assist.
In another example disclosed by Kawanishi in U.S. Pat. No. 4,835,004, web temperature control is used to reduce the non-uniformity of and preserve the level of electrical charges deposited on the web by a set of ionizers prior to the coating application locus. This uniform charge is then used to provide an electrostatic assist for the coating method to yield defect-free coatings. This patent places certain requirements on the environment (temperature, relative humidity (RH)) of the web prior to the coating application locus to achieve a uniform charge.
Thus, there is a need for a method for coating emulsions at high speeds without having air bubbles entrained in the coated layer and with no loss in adherence using electrostatic assist regardless of the presence, or absence, of surfactants in a gelatin subbing layer and without placing restrictions on the environment of the web prior to the coating application locus.